Grignard Reagents with Aldehydes, Ketones and CO2: What Alcohol or Acid You Get
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
A Grignard reagent (R-MgX) acts like a carbanion (R minus) that attacks the carbon of a C=O group. What you get depends on the starting carbonyl: formaldehyde (HCHO) gives a primary (1°) alcohol, any other aldehyde gives a secondary (2°) alcohol, a ketone gives a tertiary (3°) alcohol, and carbon dioxide (CO2) gives a carboxylic acid. Memory hook: "Zero-One-Two carbons around C=O give 1°-2°-3° alcohols, and CO2 gives an acid."
The class of product depends on the carbonyl: formaldehyde gives a 1° alcohol, other aldehydes 2°, ketones 3°, and CO2 gives a carboxylic acid, always after the H3O+ hydrolysis step.
Your doubts, answered
Which alcohol does a Grignard reagent give with each carbonyl?
Count the carbon groups already attached to the C=O carbon (not counting the O). Formaldehyde HCHO has zero carbon groups, so R-MgX gives a 1° (primary) alcohol. Any other aldehyde (R'CHO) has one carbon group, so you get a 2° (secondary) alcohol. A ketone (R'COR'') has two carbon groups, so you get a 3° (tertiary) alcohol. The new R from the Grignard becomes the extra group on the carbon that used to be C=O.
Why does formaldehyde give a primary alcohol but acetone gives a tertiary alcohol?
The Grignard adds one new R group to the carbonyl carbon. In HCHO the carbonyl carbon has 2 H atoms and no other carbon, so after adding one R it has 1 carbon neighbour = primary alcohol. In acetone (CH3-CO-CH3) the carbonyl carbon already has 2 methyl groups; adding a third R gives a carbon bonded to 3 other carbons = tertiary alcohol. So the number of carbons already on C=O decides the class.
What does a Grignard reagent give with CO2 (dry ice)?
CO2 has a C=O that the carbanion attacks. R-MgX adds to CO2 to form R-COO-MgX (a carboxylate salt). After acidic work-up (H3O+) you get R-COOH, a carboxylic acid with ONE more carbon than the original alkyl halide. This is a key way to prepare carboxylic acids and add a carbon to the chain.
Why is the H3O+ (hydrolysis) step needed at the end?
When R adds to C=O, the oxygen becomes an alkoxide bonded to MgX (an ionic salt, R-O-MgX). This is not yet an alcohol. Adding water or dilute acid (H3O+) protonates the oxygen to give the neutral -OH group. So the product is only formed AFTER hydrolysis. NEET options often trick you by giving the alcohol only if you remember this second step.
Does the R of the Grignard become part of the alcohol carbon or the OH carbon?
The R group bonds to the carbon that was the C=O carbon, and the OH ends up on that same carbon. So both the old carbonyl carbon and the new R group end up joined, with OH sitting on the carbon. Example: CH3MgBr + HCHO gives CH3-CH2-OH (ethanol), where the CH3 came from the Grignard and the CH2-OH came from formaldehyde.
Why must Grignard reactions be done in dry ether with no water?
A Grignard reagent is a very strong base. If any water, alcohol, or acidic H (like -OH, -NH, -COOH) is present, the carbanion grabs that H and turns into a plain alkane (R-H), destroying the reagent. That is why the medium is dry ether and water is added only at the very END for the hydrolysis step.
⚠️ The NEET trap ✗ Acetone + CH3MgCl gives isopropyl alcohol (a secondary alcohol), because acetone already looks like propan-2-ol. ✓ Acetone + CH3MgCl (then H3O+) gives tert-butyl alcohol (2-methylpropan-2-ol), a TERTIARY alcohol. The methyl from the Grignard adds a third carbon to the carbonyl carbon: (CH3)2C=O becomes (CH3)3C-OH. 🧠 Ketone + Grignard ALWAYS gives a 3° alcohol. Count the carbons on C=O AFTER adding R, not before.
Real NEET questions
NEET 2020
The reaction between acetone (CH3COCH3) and methylmagnesium chloride (CH3MgCl) followed by hydrolysis gives
A · tert-butyl alcohol ✓
B · isobutyl alcohol
C · isopropyl alcohol
D · sec-butyl alcohol
Solution: The methyl carbanion (CH3-) of the Grignard adds to the carbonyl carbon of acetone. Acetone already carries two CH3 groups on the C=O carbon; the new CH3 makes it three, so after H3O+ work-up we get (CH3)3C-OH. A ketone + Grignard always gives a tertiary alcohol, here 2-methylpropan-2-ol = tert-butyl alcohol. Correct answer: (A).
NEET 2021
What is the IUPAC name of the product? CH3COCH3 --(i) C2H5MgBr, dry ether (ii) H2O, H+--> Product
A · pentan-3-ol
B · 2-methylbutan-2-ol ✓
C · 2-methylpropan-2-ol
D · pentan-2-ol
Solution: Ethylmagnesium bromide (C2H5-) adds across the C=O of acetone. The carbonyl carbon (with two CH3 groups) gains the ethyl group; after acidic work-up the alkoxide is protonated to the tertiary alcohol (CH3)2C(OH)C2H5. Naming: a 4-carbon main chain (butane) with OH and a methyl on C2 = 2-methylbutan-2-ol. Correct answer: (B).
NEET 2025
The major product when C6H5-CO-CH2-CH2-CN is treated with (i) CH3MgBr (excess) then (ii) H3O+ is:
A · C6H5-CH(OH)-CH2-C(CH3)2-OH
B · C6H5-CO-CH2-CH2-CO-CH3 (a diketone)
C · C6H5-C(CH3)(OH)-CH2-CN
D · C6H5-C(CH3)(OH)-CH2-CH2-CN ✓
Solution: The Grignard CH3MgBr adds to the reactive ketone C=O (the -CO- next to the phenyl ring). The methyl carbanion joins that carbon; after H3O+ work-up it becomes a tertiary alcohol carrying a new CH3: C6H5-C(CH3)(OH)-. The rest of the chain and the nitrile (-CN) group are kept, so the product is C6H5-C(CH3)(OH)-CH2-CH2-CN. Correct answer: (D).
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the general product of a Grignard reagent with an aldehyde or ketone?
An alcohol. The alkyl/aryl group of R-MgX adds to the carbonyl carbon, and after hydrolysis with H3O+ an -OH forms on that carbon. Aldehydes give 1° or 2° alcohols; ketones give 3° alcohols.
How do you get a carboxylic acid from a Grignard reagent?
React R-MgX with CO2 (dry ice) to form R-COO-MgX, then add H3O+ to get R-COOH. The acid has one more carbon than the original alkyl halide, so this is a chain-lengthening method.
Why does a ketone always give a tertiary alcohol with a Grignard reagent?
A ketone's carbonyl carbon is already bonded to two carbon groups. When R from the Grignard adds as the third carbon group, that carbon becomes bonded to three carbons, which is the definition of a tertiary alcohol.
What happens if water is present during the Grignard reaction?
The Grignard reagent is destroyed. It is a strong base and reacts with the acidic H of water to become an alkane (R-H). That is why the reaction is run in dry ether and water is added only for the final hydrolysis.
Is this topic important for NEET?
Yes. Grignard reactions appear almost every year in the Aldehydes, Ketones and Carboxylic Acids chapter (2020, 2021, 2025 all had one). Questions test which class of alcohol forms and the IUPAC name of the product, so knowing the 1°/2°/3° rule scores easy marks.
